It is conventional to desalt or dehydrate oil-continuous emulsions by introducing such emulsions directly into an electric treating field of sufficient intensity to coalesce the suspended droplets of the dispersed phase into massses of sufficient size to gravitate from the oil. The dispersed phase of such emulsions is composed of a material, usually aqueous, that is sufficiently immiscible with the oil to produce an internal or dispersed phase. Initially, the dispersed droplets are of such small size or are so stabilized that they will not readily gravitate from the oil phase. However, the electric field coalesces such dispersed droplets, and it is found that the resulting coalesced masses gravitate rapidly from the oil, usually in the same container as that in which electric treatment takes place. In a desalting operation, a quantity of water may be mixed with the incoming crude oil in a valve or other mixer, all as well known, so that a more complete removal of salt from the oil may be accomplished. Typical of such prior art dehydrator/desalters are those described in the folowing U.S. Pat. Nos.: 1,838,922, 2,527,690, 2,880,158, 3,206,686, 3,577,336, 2,033,446, 2,543,996, 2,892,768, 3,250,695, 3,649,516, 2,041,954, 2,557,847, 2,894,895, 3,458,429, 3,672,127, 2,072,888, 2,848,412, 2,963,414, 3,592,756, 3,736,245, 2,443,646, 2,855,360, 3,141,000, 3,531,393, and German Pat. No. 1,014,076 (Helmut Stock; Aug. 22, 1957).
U.S. Pat. Nos. 4,149,958 and 4,182,672 to Robert B. Martin disclose apparatus and processes for desalting or dehydrating emulsions of the type above described, wherein the apparatus has a plurality of electrified coalescing stages in a single vessel, wherein the stages are hydraulically susbstantially independent and adapted for parallel or series stage operations. The expression "electrified" denotes the presence of an electric field, as above described. In one embodiment shown by Martin, two or more electrified stages, each separately energized, are operated in parallel to proportionately increase vessel throughput capacity. In another embodiment, especially designed for desalting, series operation is employed, wherein each successive stage receives the product of the preceding stage as feed, with fresh and/or recycled water being added.
The embodiments specifically illustrated and described in the Martin patents incorporate centrally positioned emulsion inlet distributors between horizontally disposed planar permeable electrodes in vertically spaced relationship. These distributors decrease the effective space between electrodes unless the top and bottom electrodes are provided with humps to allow more clearance between the distributors and the electrodes. Alternatively, if the electrodes are not provided with humps, the top electrode would, in practice, have a hole in it which would be directly above the distributor so that the distributor, which is at ground potential, would be electrically separated from the top electrode by at least the same distance as between adjacent electrodes. A similar hole and clearance would be needed for the other electrodes.
The necessity for humps in the bottom and top electrodes and/or central holes in the electrodes creates clearance and design problems. Moreover, if the center electrode in the two stage treaters of Martin U.S. Pat. Nos. 4,149,958 and 4,182,672, with a double box distributor arrangement, is energized rather than grounded, it would require such electrode to be in two separate parts, one on each side of the distributor assembly.
Further, in Martin's specific embodiments of the series plural stage desalter, collectors at each side of the treating vessel are provided in all but the final stages. These have to be tied together hydraulically and balanced to draw equally from each side.
It is an object of this invention to provide a plural stage dehydrating/desalting apparatus, especially useful for desalting crude oil which avoids the above discussed problems of the Martin apparatus referred to.
It is a further object of this invention to provide such a dehydrating/desalting apparatus which accomodates a separate distributor for each stage with a minimum of lost electrode area and which is adapted to the use of a middle energized electrode as a single unit, which is easier to hang on insulators and charge.
It is still a further object of this invention to provide improved processes for dehydrating or desalting crude oil employing improved plural stage dehydrating/desalting apparatus.
Additional objects of the invention will be evident to those skilled in the art in the course of the following description.